Diagnostic rule #2... eliminate one thing at a time until the problem is found.

It should be easy enough to remove the washer valve from the system and know if that is the problem.

I remember reading of a similar problem somewhere and it was an automatic washer shut-off valve that caused the problem.

If the radiant heat switches on depending on it's size, it will suck the heat out of the tank quickly. Being probably only 40,000 Btus, most likely it can't keep up. Poor mans combined radiant heat with domestic hot water off one tank is a lousy and potentially unsafe system.

How old is the heater? Could sediments and very cold outside temps be a contributing factor?

Could the dip tube have fallen down?

In a tank, there is a tube that connectes to the cold water inlet and directs the cold to the bottom of the tank. The hot comes from the top of the tank. If this tube comes off, then the fresh cold that comes into the tank goes immediately out through the hot.

Disconnect the fill line to the water heater and check if the dip tube is broken.

Thanks everyone for the responses. Our water heaters are only 6 years old so its hard to believe they would be busted already. State brand high efficiency 50 gal, 65000btu heaters.

We turned off the radiant heat and isolated the system bypassing the floor heat. Same problem. I agree with some of you that it could actually be the dip tube. A funny thing though, the water return seems to come back cold. We may also have a broken check valve somewhere.

Seems the only way to fix this is to break down the parts and see whats busted. It may be a costly repair. Thanks to everyone. I post once we solve the mystery.

Success!

After a long battle, we finally figured out the problem. It turns out, there was a bad check valve, near the recirculating pump that allowed the cold water to get into the system. the same check valve failed on both systems. We opened it up, and both of them were encrusted with calcium deposits and hard water debris not letting the valve close.

Just for future reference to anybody with a similar problem here is how we tackled it:

-We checked the dip tubes on both hot water heaters , and both of them were fine.

-next, my plumbers isolated the hot water heater to make sure the heater was running properly. all checked out fine.

-We turned off the radiant heat systems and bypassed them with the built-in valves.

-My plumbers opened up each shower fixture and closed off the water valves to take those out of the equation. the hot water still ran cold.

- Next, the plumbers removed the recirculating pump and checked the system. the water still ran cold. at this point it had to be the check valves, since nothing else would cause the cold water to come back into the system.

- The plumbers then starting removing the check valves beginning with the one near the recirculating pump and discovered the broken valve. If this still didn't work, they would've kept going to check every check valve in the system(three total).

the plumbers installed a temperature gauge to be able to see the change in water temperature. now the temperature holds solid and 120� when the hot water is turned on. I can't wait to take a piping hot long shower.

Below is a picture of the plumbing system if you were curious. Two water heaters, one for each unit with the radiant heat manifolds.

Radiant heat system by Warmfloors, Napa Ca
Installation by AG Quality Plumbing San Francisco, CA(the best plumbers in San Francisco by far)

Thanks all!

Here is a link that might be useful: {{gwi:2013484}}

They weren't BAD check valves and they didn't FAIL. The fact that the check valves died in both system shouts out that it is the environment and not the check valves failing. Hard water does that and will continue to do that to fixtures, valves, appliances, and plumbing.

Ever had your water tested cause radiant heat doesn't like hard water.